Variable coupling device



A$29, 1950 R. sPRoULl. 2,520,614

VARIABLE couPLING DEVICE Filed Feb. 2s, 194e l @EME Patented Aug. 29,1950 VARIABLE `CUPLING' DEVICE Robert L. Sproull, Princeton, N. J.,assignor to Radio Corporation of 1AAmerica, a corporation of DelawareApplication February 26, 1946, Serial No. 650,369 v 2 Claims. (Cl.178-44) This invention relates generally to microwave i apparatus andmore particularly to improved variable-coupling wide-frequency-bandtunable microwave resonators of the type utilizing a movable pistonwherein adjustment of thel piston for conductive piston enclosed withinthe resonator shell. Well known methods of coupling into or out. of suchadjustable resonators include the use of microwave probe elements,coupling loops and adjustable aperture devices, depending uponA thecharacteristics of thev circuit to which the resonator is coupled. It.is frequently advantageous tobe able to vary the degreey ofcouplingbetween an external circuit and the microwave resonator as theresonator is, tuned through its useful. frequency band. In particular inreex oscillators of the type including an electron.v reflectingelectrode, it is desirable that. the coupling coefficient between theoscillator and the load circuit remain substantially constant ordecrease as the oscil lator frequency is increased. A conventional cou-'pling loop, probe or aperturey device. coupled into the oscillatorcavity normally will provide an increased coupling coeiiicient as thefrequency is increased, thereby providing non-linear loading of the,oscillator over its useful frequency range.

The instant invention is related toand comprises anl improvement overthe devices disclosed in the copending U. S. application of Ernest G.Linder, Serial No. 650,719; filed February 27, 1946, covering severalembodiments of microwave appaifa-tus wherein the frequency 0f, amicrowave. resonator is varied by longitudinal'adjustment of a pistonoperative. therein, and whereinthe effective length of a microwave,coupling probe, or the originar-displacement of a coupling loop aswellas` the effective opening of an aperture device is simultaneouslyadjusted by movement of the tuning piston. The invention asl disclosedtherein provides means for tuning a. microwavegenerator andsimultaneously adjusting the coupling thereto from an external coaxialline or Waveguide device.

In contradistinction thereto, the instant invention comprises avariable-coupling wide-frequency-band microwave coaxial lline resonator2 A including a movable piston having an aperture therein disposedwithin and between the resonator conductors. A coupling loop of uniformcross.- section terminates a coaxial line connected to an externaldevice. The loop extends through the resonator and through the aperturein the tuning piston. Longitudinal adjustment of the tuning piston thuscontrols the resonatorI frequency and the coupling coefficient of theloop in opposite sense. Since there is no contact between the loop andthe movable piston, the commonly erratic characteristics of movablecontacts at microwave frequencies is eliminated insofar as coupling isconcerned. A4 modification of the invention utilizes a coupling loophaving a'. tapered crosssection wherein the coupling coefficient changesin accordance with a predetermined non-linear function of the resonantfrequency of the resonator.

Among the objects of the instant invention are to provide an improvedmethod of and means for simultaneously adjusting the tuning and couplingof a microwave resonator device. Another object of the invention is toprovide a method of' and means for tuning a coaxial microwave resonatorover a relative wide frequency range and for simultaneously adjustingthe coupling coefficient' of a coupling element connected to an externalcircuit. A further object of the inveni tion is to provide an improvedcoaxial microwave resonatory including a longitudinally adjustabletuning piston, wherein adjustment of the pistonV simultaneously adjuststhe effective coupling to the resonator of a `microwave coupling loop.Another object is to provide an improved' coaxial' microwave resonatorincluding a longitudinally adjustable tuning piston, wherein adjustmentof the piston simultaneously adjusts the effective coupling of amicrowave coupling loop having a predetermined" tapered conformation forcoupling an external circuit to said resonator according to aVpredetermined function of the operating frequency;

An additional object is to providev an improvedI variable-coupling widefrequency' band' microwave resonator including a frequency adjustingpiston wherein adjustment of the tuning piston simultaneously varies inopposite sense the reso nant frequency of said' resonator andthecoupling coefficient of a coupling loop coupling said resonator to anexternal circuit. A further object is to provide a constant-couplingwide-frequencyband microwave resonator including a movable tuning pistonwherein adjustment of the tuning piston varies the resonant frequency ofsaid resoelevational View of a second embodiment of the invention.Similar reference charactersV are applied to similar elements throughoutthe drawing. l

Referring to Figures 1 and 2 of the drawing, a typical microwaveresonator according to the invention includes inner and outer coaxiallydisposed conductors I, 3, wherein the outer conductor includes a closedend portion 5. The resonator is tuned by means of a longitudinallyadjustable piston 'l having an aperture 9 disposed therein in the regionintermediate the inner and outer coaxial line conductors. adjustment ofthe piston 1 may be accomplished by means of actuator bars IB connectedto any desired type of actuating mechanism not shown.

Y The piston 'l includes a pair of cylindrical slotted contact elementsIl, i3, which contact the outer and inner resonator conductors,respectively, to insure uniform electrical contact with the piston.

A second coaxial line having an outer conductor I5 and an innerconductor l1 is terminated withinthe tunable resonator by means of ahairpin coupling loop I9. The coupling loop I9 extendsthrough theportion of the resonator intermediate the end face 5 and the tuningpiston 1 and also extends through the aperture 9 in the tuning piston'1.

Thus, as the resonant frequency of the resona- 1 tor l is increased byadvancement of the piston face toward the end resonator wall 5,increasingly shorter portions of the coupling loop are exposed to theresonator cavity, thereby correspondingly tending to reduce the couplingbetwen the resonator and the external coaxial line i5, l?. The

The longitudinal sense to the variation of the resonant frequency of theresonator as the piston position is adjusted.

It should be understood that the variableor constant-coupling systemsdisclosed herein may be employed as either output or input couplingelements for associated circuits and that simultaneous or separatecontrol of a plurality of such coupling elements may be provided for asingle resonator or for a plurality of resonators. By properproportioning of the adjustable resonator and the associated couplingloop or loops, the coupling coefcient to external circuits may bekmaintained substantially constant or varied in opposite sense to theadjustment of the resonator frequency.

Thus, the invention disclosed comprises several embodiments of coaxialresonator wide-frequency-band microwave devices providing tuning andfrequency variation by adjustment of a tuncoupling from the maskedportion of the loop through the aperture B to the resonator cavity ispractically negligible. Depending upon the circuit parameters, thecoupling coeflicient to an external circuit, not shown, may bemaintained substantially constant throughout the useful frequency rangeof the resonator, or the coupling coi e'icient may be reduced as thefrequency is .in-

creased. Since the loop does not contact the movable piston and is xed`with respect to the resonator walls, erratic operation due to varyingcontact resistances is minimized.

The'device illustrated in Figure 3 is similar to that described Vbyreference to Figures 1 and 2,

with the exception that the coupling loop i9 is of non-linearcross-section, whereby longitudinal adjustment of the tuning piston ivaries the coupling coelcient of the coupling loop I8' according to anon-linear function of the operating frequency of the resonator. Itshould be understood that the shape of the coupling loop l 9 Vmay bevaried in accordance with the desired coupling coefficientcharacteristic to provide either constant coupling or variable `couplingin opposite ing piston, wherein such piston adjustment providescorresponding control of the coupling coeincient between said resonatorand an external circuit. Y

I claim as my invention:

1. A constant coupling wide-frequency-band tunable microwave resonatorincluding a coaxial line section, means including a movable pistonhaving an aperture therethrough disposed within andsubstantially'conforming to rsaid section and comprising with said linesection said tunable resonator, and a, U-shaped coupling loop extend#ing into said resonator through the wall of said resonator opposite saidpiston andv having a U-shaped portion'of said loop extending throughsaid aperture in said piston for microwave coupling to said resonator,whereby adjustment of said tuning piston varies the reso-nant frequencyof said resonatorand the'coupling coefhcient to said resonator of saidcoupling loop in predeter- `rnined relative sense and degree.

2. A constant coupling wide-frequency-band tunable microwave resonatorincluding a coaxial line section, means including'a movable pistonhaving anaperture therethrough disposed Within and substantiallyconforming to said section and comprising with said line section saidtunable resonator, anda U-shaped coupling loop of predetermined taperedwidth extending into said resonator through the Wall of said resonatoropposite said piston and having a U-shaped portion of said loopextending through said aperture in said piston for microwave coupling tosaid resona-f tor; whereby adjustment of said tuning piston varies theresonant frequency of said resonator Without substantial variation ofthe coupling coeilioient to said resonator of said coupling loop.

ROBERT L. SPROULL.

REFERENCES CITED The following references are of record in the ille ofthis patent:

l UNITED STATESv PATENTS

